An Optimized machine for fast grape cooling by cryogenic fluid

A plant for the rapid cooling of a quantity of fresh grapes to be sent to a mash-destemmer machine has been designed. It uses a cryogenic fluid: liquid CO2. The cooling time for the considered plant is about 8 s. The machine works with a grape flow of approximately 2 ÷ 3 tons/h, with an adjustable thermal jump (Tinlet – Toutlet) of 20 K. This machine is suitable in the cryomaceration process of the grapes. The cryomaceration process can also be performed using traditional, large capacity refrigeration units; they are very expensive, and very often damage the cooled product, due to the friction generated by the passage in the pipes, causing the formation of dregs that reduce the quality of the final product. This is avoided by the considered machine. It consists of: a vibrating table, that allows the stratification of grapes producing only one layer of grapes cluster; infrared temperature transducer that allow to determine the inlet grapes temperature; nozzles for the injection of liquid CO2; insulated belt bucket elevators, that allow to sent the grapes to the mash-destemmer without thermal exchanges; mash-destemmer machine acting in a CO2 inert environmental and at the maceration temperature. The process is managed by a PLC that, by setting the input CO2 quantity, check the process temperature. The CO2 quantity used depends, naturally, on the process temperature jump. In order to optimise the cooling process, a computational fluid dynamics (CFD) model of a tunnel for grapes cryomaceration has been developed. The current study has focused on understanding the main thermal phenomena, such as the conduction and convection heat transfer, aimed to the cooling plant optimisation. Simulations were carried out by a CFD code of the ANSYS package, CFX11. Because of symmetry, only half of the model was simulated. Initial analyses were carried out to test the goodness of the mesh. The simulations were performed on several models to investigate the plant efficiency with different grapes sizes and, moreover, the necessary CO2 flow, as a function of the initial temperature of the grapes. CFX predictions were used to investigate the behaviour of nozzles which inject liquid CO2 in the direction of product flow from a vibrating conveyor, so as to increase the thermal exchanges and to optimise the system performances. CFD analysis has allowed to predict the necessary CO2 flow to achieve the desired final temperature and to obtain an optimized machine for the fast grape cooling.